In an end-address microphone (a rod-shaped microphone that picks up sound waves from its one end), a rear acoustic terminal is made by forming an opening in a peripheral wall portion of a cylindrical unit case. An example (see, for example, Japanese Patent Application Publication No. 2011-009807) of the conventional technique is described with reference to FIG. 4.
A unidirectional condenser microphone unit (hereinafter, simply referred to as “microphone unit” in some cases) 1B includes a cylindrical unit case 10 made of an aluminum material, a brass alloy, and the like.
The microphone is an end-address microphone, and hence one end (the left end in FIG. 4) of the unit case 10 is opened as a front acoustic terminal 11. An opening part as a rear acoustic terminal 12 for taking in velocity components is formed in a peripheral wall portion of the unit case 10 that is away by a predetermined distance from the front acoustic terminal 11.
Note that a lock part 10a bent inward is formed in an opening part of the front acoustic terminal 11. A female screw 10b is formed at another end (rear end) of the unit case 10. Moreover, in this example, a guard net 11a made of a wire net or the like is attached to the opening part of the front acoustic terminal 11.
Normally, some linear protrusions or grid-like rails 12a are provided in the opening part of the rear acoustic terminal 12, mainly from the viewpoint of designing. A dust-proof guard net 12b made of, for example, a wire net is placed on the inner side of the rails 12a. 
The microphone unit 1B includes an electrostatic type electroacoustic transducer 20 in which a diaphragm 21 and a fixed pole 24 are opposedly placed with the intermediation of an electrically insulating spacer ring 23, the diaphragm 21 being stretched on a support ring (diaphragm ring) 22 at a predetermined tension, the fixed pole 24 being supported by an electrically insulating seating 25 made of, for example, synthetic resin.
A plurality of sound holes (holes for allowing sound waves to pass therethrough) 25a are pierced in the insulating seating 25. Although not illustrated in detail, the fixed pole 24 is made of a porous plate, and includes a plurality of sound holes. In this example, an acoustic resistance material 26 made of a felt material and the like is placed between the fixed pole 24 and the insulating seating 25.
The electroacoustic transducer 20 is housed between the front acoustic terminal 11 and the rear acoustic terminal 12 in the unit case 10. A sealing member 30 is fitted to the another end (rear end) of the unit case 10. As a result, an air chamber A having a predetermined volume for obtaining non-directional components is formed on the back surface side of the insulating seating 25. In this example, a cylindrical member whose apex part that faces the insulating seating 25 is formed in a truncated conical shape is used as the sealing member 30.
A sealing substrate 40 is attached to the rear end of the sealing member 30, and a lock ring 50 is screwed into the female screw 10b formed at the another end of the unit case 10. As a result, the electroacoustic transducer 20 is fixed in the unit case 10 by the sealing substrate 40 and the sealing member 30 while abutting against the lock part 10a. 
Note that an electrode draw-out terminal 27 of the fixed pole 24 is drawn out from a central portion of the insulating seating 25. A relay terminal 31 in fitting contact with the electrode draw-out terminal 27 is provided in the central portion of the sealing member 30 so as to penetrate through the sealing substrate 40.
Moreover, the microphone unit 1B is coupled to a cylindrical microphone grip (microphone main body) (not illustrated) with the intermediation of, for example, the female screw 10b, and the relay terminal 31 is electrically connected to a circuit board including a speech signal output circuit housed in the microphone grip.
In the microphone unit 1B, sound waves that have entered the microphone unit 1B from the rear acoustic terminal 12 act as velocity components on the back surface of the diaphragm 21 through the air chamber A, the sound holes 25a of the insulating seating 25, the acoustic resistance material 26, and the sound holes (not illustrated) of the fixed pole 24. As a result, the microphone unit 1B operates in a unidirectional manner.
Meanwhile, microphone units are generally designed on the basis of lumped constant equivalent circuits, assuming planar waves. In the case of a high frequency of around 10 kHz, however, acoustic elements approach the wavelengths of sound waves, and hence the lumped constant equivalent circuits are insufficient.
At a frequency at which a half wavelength ½λ, of a sound wave is equal to the distance between the acoustic terminals of the electroacoustic transducer 20, a sound pressure gradient between the front acoustic terminal 11 and the rear acoustic terminal 12 disappears. Hence, the drive force of the diaphragm 21 based on the sound pressure gradient cannot be generated. For these reasons, the directionality in a high tone range of the unidirectionality (polar pattern, cardioid) deteriorates.
Moreover, portions of the sound holes 25a of the insulating seating 25 are defined as acoustic resistance parts AR behind the fixed pole, and a sound path (sound wave path) running from the acoustic resistance parts AR to reach the rear acoustic terminal 12 is observed. In this case, in the above-mentioned conventional technique, because the apex part of the sealing member 30 that forms the air chamber A has the truncated conical shape, the cross-sectional area of the sound path rapidly changes when the sound path is viewed from the acoustic resistance parts AR side.
As a result, the sound path has an impedance containing reactance, the narrow portion operates as acoustic mass (represented by an inductance L in an equivalent circuit), and the rapidly widened portion operates as acoustic capacitance (represented by a capacitance C in the equivalent circuit). Hence, resonance occurs in a given sound range, and a directional frequency response deteriorates.
In this way, in the microphone unit 1B according to the above-mentioned conventional technique, the sound path running from the acoustic resistance parts AR to reach the rear acoustic terminal 12 is discontinuous, and the rear acoustic terminal 12 is formed as a mere opening and does not have acoustic mass. For these reasons, as illustrated in a graph of a directional frequency response in FIG. 3B, the frequency response at 0 degrees has a peak around 7 kHz, and the frequency response at 180 degrees rises therearound, so that the directionality deteriorates.
In view of the above, the present invention has an object to set an acoustic impedance on a rear acoustic terminal side in a unidirectional condenser microphone so as to obtain satisfactory directionality even in a high tone range.